Certain embodiments of the present invention generally relate to methods and apparatus for heat treating electrical contacts and, more particularly, for induction heating plated spring type micro contacts mounted in a substrate.
Electrical components are constructed today with numerous types of electrical contacts for varied applications. In certain applications, components such as processors and the like are plugged into sockets that are mounted on a circuit board. Contacts on the component are joined with contacts in the socket or on the circuit board. As technology advances, the size of both the components and the contacts decreases. In addition, it has become increasingly important to locate more contacts in a smaller area on the component, socket and circuit board, while improving the signal performance characteristics.
Certain applications use a socket and component combination that permits the component to be removed and/or replaced periodically from a circuit board or another power and data signal carrying structure. The components and sockets are formed with corresponding arrays of mating contacts. By way of example, the array of contact in the socket may be spring contacts that flex to form a mating interface with the mating contacts of the components.
Contacts are formed from a variety of processes and materials depending upon the characteristics that the contact must possess. For certain applications, the contacts are constructed with a core wire that is highly conductive, such as a gold wire, where the core wire is plated with an alloy material, such as nickel and the like that provides the strength of the contact, such as through plating of nickel alloy and the like. The contact may further be plated with another alloy material, such as gold alloy, to enhance the electrical contact properties and/or for corrosion protection. The core wire may be coated through any of a variety of plating processes, such as sputtering, electroplating, electroforming, chemical vapor deposition and the like. Once the core wire is coated, a plated electrical contact is formed.
However, once coated, the contact may experience unstable mechanical properties that break down at elevated temperatures. In particular, the plating process forms a series of coating layers that have a layered microstructure. The layered microstructure after plating resides in a non-equilibrium state. The layered microstructure of the nickel alloy and the like exhibits internal stresses within and between the coating layers of the contact. These internal stresses are also referred to as “residual stresses.” The internal stresses increases the overall strength properties of the contact. However, the internal stresses cause the contact to exhibit inferior stress-relaxation properties when the contact becomes heated. The stress-relaxation properties refer to the ability of the contact over a period of time to maintain the required contact normal force and/or to return to and retain its original shape after the contact is placed under a load during numerous operation cycles. It is desirable to maintain high normal forces when the contact is placed under a load to ensure low contact resistance during use.
The stress-relaxation properties should be considered in applications where contacts in a socket mate with contacts in a component. The mating contacts are placed under a load that bends the contacts. During operation, the contacts carry power or data signals which creates a certain amount of heat. The contacts are also heated by heat transfer from the surrounding electrical components. When the layered microstructure of the contact is heated, the internal stresses within the microstructure cause the microstructure to realign or recrystalize in an attempt to reach an equilibrium state. If the microstructure is recrystalized to an equilibrium state while in a loaded and bent position, the contact loses the ability to maintain the required contact normal force and/or to return to its original unloaded shape. Hence, the contact exhibits an inferior stress-relaxation property in that the force exerted by the socket contact upon mating with a component contact is reduced which leads to an inferior connection and poor signal performance.
In the past, once the contacts were plated, the contacts were heat treated prior to use in order to improve the stress-relaxation properties. The heat treatment process, also referred to as annealing, involves heating the contacts, after plating, to an elevated temperature for an extended period of time. Annealing enables the microstructure to recrystalize and reorganize into an equilibrium state, meanwhile relieving the internal stresses. The annealing process is carried out while the contact is without a load and therefore the contact remains in its original un-bent shape. Subsequent heating of the contact during use does not cause further recrystalization and thus does not degrade the stress-relaxation properties of the contact.
In the past, ovens have been used to anneal contacts that have been electroplated with nickel. In order to ensure that the oven relieves the internal stresses within the contact, the annealing process continued for a relatively long period of time at a relatively high temperature. For example, to anneal wrought or cast nickel alloys, the oven may be heated to 700° C. for hours.
In certain applications, the contacts are preformed or loaded onto a substrate before the annealing process is carried out. Consequently, the substrate must be able to withstand the temperatures in the oven for the period of time set for annealing. The substrate must be composed of materials that are capable of withstanding the annealing process. The options for substrate materials are limited and therefore relatively expensive. Accordingly, the oven can only be heated to a temperature that the substrate can withstand. In applications heating the substrate, the oven cannot be heated to 700° C. since even high grade substrates break down at such high temperatures. An annealing process is needed that enables lower temperature substrates to be used with the contacts.
In addition, conventional annealing processes utilize isothermal ovens that maintain a uniform temperature throughout the oven. Consequently, as contacts are heat-treated in the oven, the entire contact is uniformly heated. While the annealing process improves the stress-relaxation properties of the contact, the annealing process somewhat reduces the overall strength of the contact. Consequently, ovens that uniformly heat the entire contact equally reduce the strength of the entire contact.
In certain applications, it would be advantageous if different portions of the contact exhibit different mechanical properties. For example, certain portions of a contact may undergo a majority of the bending or flexing within the contact, while other portions of the contact do not bend at all. Consequently, the portions of the contact that bend should exhibit desirable stress-relaxation properties; that is, the bending portion of the contact should be able to provide the required contact normal force and/or to return to its original shape even after numerous mating and unmating cycles. Other portions of the contact may never bend, yet experience a significant amount of stress as the contact is placed under a load. For instance, the base portion of a contact may never bend, but it will experience substantial stress where the base portion secures the contact to a connector, substrate or other structure. It is preferable that the portion of the contact experiencing the greatest stress exhibit superior strength properties, with less concern for the stress-relaxation properties in this particular portion of the contact. Otherwise, the base portion may fracture and experience cracking during numerous mating and unmating cycles. Conventional annealing processes uniformly heat the contacts and thus the entire contact exhibits common strength properties and common stress-relaxation properties.
A need exists for a method and apparatus to anneal certain portions of a contact more than other portions of the same contact. A need remains for an improved heat treatment process and an apparatus that overcomes the disadvantages noted above and experienced in the prior art.